Images are formed on flat panel displays, such as those used in televisions or notebook computers, by electrically controlling the optical properties of a large number of individual picture elements, or "pixels," made of an electro-optical material, such as a liquid crystal material. The large number of pixels allows the formation of arbitrary information patterns in the form of text or graphic images. The optical state of each pixel, which depends upon the voltage present across it, is controlled by applying electrical signals to addressing electrodes. The number of electrodes necessary to address the large number of pixels is greatly reduced by having each electrode address multiple pixels. In one common embodiment, transparent electrodes are positioned on opposing inner surfaces of parallel, transparent plates. A matrix of pixels is typically formed by electrodes arranged in horizontal rows on one plate and vertical columns on the other plate to provide a pixel wherever a row and column electrode overlap. Addressing signals determined by the image to be displayed are placed onto the electrodes by addressing signal voltage drives. A typical liquid crystal display may have 480 rows and 640 columns that intersect to form a matrix of 307,200 pixels. It is expected that matrix liquid crystal displays may soon comprise several million pixels. Because different pixels are addressed by the same electrodes, the optical states of individual pixels can be incidentally affected by the optical states of other pixels addressed by the same electrode.
The voltages of the addressing signals applied across the column electrode ("column signals") are typically dependent upon the image to be displayed, whereas the voltages of the addressing signals applied across the row electrodes ("row signals") are typically image independent. For example, in the Alt and Pleshko matrix addressing system described in "Scanning Limitations of Liquid Crystal Displays," IEEE Trans., ED21, pp. 146-55 (1974), each image-independent row signal effectively consists of a single rectangular pulse, the rectangular pulses being applied sequentially to each of the row electrodes. Every row is pulsed or "selected" during an addressing interval, once in each frame period. Typically there are as many addressing intervals as there are display rows.
Another technique for matrix addressing, known as "Active Addressing.TM.," is described in co-pending U.S. patent application Ser. No. 07/678,736 of Scheffer et al. for "LCD Addressing System." In the Active Addressing.TM. technique, the row signals comprise more than one selection pulse, the multiple selection pulses being distributed over the frame period. These row signals are preferably a set of orthonormal functions, such as Walsh functions. The column signals during each addressing interval depend upon the desired image and the row signals. The frame period is divided into multiple addressing intervals, with multiple rows being selected during any addressing interval.
In a passive matrix-type display, neglecting electrode resistance, the voltage across a pixel at any time is the difference between the voltages on the row and column electrodes that define the pixel. The pixel voltage varies during a frame period. The pixel voltage over a frame period can be characterized by a pixel voltage waveform having a root-mean square ("rms") value. Addressing systems are directed toward controlling the rms voltage across a pixel.
The optical state of a pixel, i.e., whether it will appear dark, bright, or an intermediate gray shade, is determined by the orientation of the liquid crystal molecules associated with the pixel. For a supertwisted nematic device, the orientation of the liquid crystal molecules is altered by modifying a voltage applied across the pixel. The applied voltage produces on the liquid crystal molecule an electric torque that is proportional to the square of the voltage. The electric torque counters an elastic torque and a viscous torque, the magnitudes of which determine a characteristic response time for the pixel. Therefore, if the characteristic response time of the pixel is many times longer than the frame period, the optical state of a pixel will be determined primarily by the rms voltage across the pixel, averaged over the frame period. To change the optical state of a pixel, the rms voltage across that pixel must be appropriately changed.
In a typical matrix-type display, all of the pixels in a column share a common column electrode, so changing the rms voltage across one pixel might change the rms voltage across the other pixels in the column as well. However, by implementing an addressing technique known as "multiplexing" it is possible to change the rms voltages (within certain limits) of individual pixels in the column without affecting the rms voltages across the other pixels. By applying the multiplexing technique, the optical states of individual pixels in the column are changed by appropriately changing the addressing waveform applied to the column electrode. This change affects the pixel voltage waveform of all of the pixels in the column but does not necessarily change the rms voltage across those pixels because many different waveforms can have the same rms value averaged over a frame period. The art of multiplexing lies in generating and applying the appropriate waveforms to the columns such that predetermined rms voltages are produced across each pixel in the columns.
However, it is known that the optical response of a liquid crystal display to an applied electrical signal depends not only on the rms voltage value of the signal but also on the frequency of that signal if it is a sine wave and on the complete frequency spectrum of the signal if it is a more complex waveform. Thus, to precisely predict the optical response of a pixel to an applied signal, the amplitudes of each frequency component of the signal must be known in addition to the rms value of the signal.
The pixel voltage waveform is determined primarily by the addressing signals applied to the row and column electrodes that overlap to define the pixel. The row and column signals can be separately analyzed into spectral voltage components, which contribute to the spectral makeup of the pixel voltage waveform. The column signals present on the column electrodes over a frame period depend upon the optical state of all pixels in the column. The optical state of each pixel depends, therefore, upon the optical states of all other pixels in the column.
In some displayed images, this interdependence results in a phenomena known as "ghosting," which is manifested by vertical trails above and below written characters on the display. Ghosting is especially noticeable when OFF pixels in one column have a slightly different optical state than adjacent OFF pixels in columns having different information patterns. This occurs because, although the rms voltage across all of the OFF pixels is the same, the spectral composition of each of the pixel voltage waveforms can be different, depending upon the displayed information of each column; some OFF pixels will have waveforms with lower or higher spectral voltage components than other OFF pixels, causing those pixels to have different optical states. We will refer to this type of crosstalk as "spectral crosstalk."
The dependence of the optical state of a pixel on the frequency of the spectral components of the pixel voltage is a consequence of both the device characteristics and the material constants of the display. The device characteristics can cause the actual pixel voltage, i.e., the potential difference across the electrodes at the pixel, to be different from the applied pixel voltage, i.e., the potential difference applied by the signal at the corresponding addressing electrodes. For example, the sheet resistance of the display electrodes and the capacitance of the liquid crystal layer are known to act together as a distributed low-pass filter. Thus, the high frequency components of the addressing signals are more attenuated than are the low frequency components, as shown by actual voltage measurements at the pixel site. This attenuation distorts the waveform appearing across the pixel from the waveforms placed onto the electrodes by the signal driver and decreases the actual rms pixel voltage. This attenuation is especially strong when a significant proportion of the frequency components of one or both of the addressing signals occur at higher frequencies. FIG. 1 is a graph showing the relative brightness of the optical state of a pixel versus frequency for a typical nematic liquid crystal display with sine-wave drive at constant rms voltage. The roll-off at high frequencies is primarily caused by this attenuation effect. The increase at low frequencies is believed to be caused by interfacial double layers, which are believed to be generated by ionic impurities.
The liquid crystal material constants also contribute to the frequency dependence of the optical state of the pixel. The effect of an applied electric field on the orientation of the liquid crystal molecules is determined in large part by the dielectric anisotropy, i.e., the difference between the dielectric constant of the liquid crystal measured parallel and perpendicular to the long axes of the molecules. The dielectric anisotropy is not a constant; it is a function of the frequency of the voltage across the liquid crystal. Therefore, the orientation of the liquid crystal and its associated optical state are also frequency dependent. Part of the roll-off at high frequencies shown in FIG. 1 is due to this effect. This frequency dependence also causes column ghosting as described above.
Addressing systems have been designed primarily to apply a desired rms voltage across a row and column electrode during a frame period. The frequency dependence of the optical state has been minimized primarily by attempts to modify the liquid crystal material constants and device parameters to weaken the dependence of the optical state on frequency. See, for example, Akatsuka et al., Material Approach for Reduction of Cross Talk in Simple Matrix LCDs," IEEE (1991). Akatsuka describes reducing the dependence of threshold voltage on frequency by increasing the resistivity of the liquid crystal material and of the alignment layer. Higher resistivity liquid crystal materials and alignment layers are, however, difficult to maintain in a display cell over long periods of time due to the slow contamination caused by impurities diffusing into the cell through the edge seal.